cfg80211: fix CONFIG_CFG80211_EXTRA_REGDB_KEYDIR typo

The kbuild change here accidentally removed not only the
unquoting, but also the last character of the variable
name. Fix that.

Fixes: 129ab0d2

 ("kbuild: do not quote string values in include/config/auto.conf")
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20220221155512.1d25895f7c5f.I50fa3d4189fcab90a2896fe8cae215035dae9508@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>